1. Field of the Invention
The invention relates to a steam generator having a plurality of heat-exchange areas for the preheating, evaporation and superheating of the working medium. It relates in particular to the steam-temperature control inside the steam generator.
2. Discussion of Background
Steam generators in the form of heat-recovery boilers for utilizing the waste heat of the gas turbines are as a rule made with a drum-type boiler, with a once-through boiler or with combinations thereof. Compared with conventionally fired boilers, however, the flue-gas temperature in heat-recovery boilers is limited. In particular the evaporation, which takes place at constant temperature, leads to thermodynamic and technical design constraints. In order to achieve as high a flexibility and adaptability to the profile of the flue-gas temperature as possible, the following measures have been proposed hitherto:
An increase in the number of pressure stages; up to 3 pressure stages with reheating are not uncommon. PA1 A complicated division of the heating surfaces in the upper range of the flue-gas temperatures; e.g. alternate arrangement of superheater and reheater heating surfaces. PA1 Branching and merging of feedwater flows or steam flows.
The intention is thus to achieve high live-steam and reheater temperatures. The advantage of a low temperature-difference rating at the pinch point entails considerable complexity as regards heating surfaces.